Generally, an image sensor is a semiconductor device that converts an optical image to an electric signal and can be categorized into a charge coupled device (CCD) and a CMOS image sensor (CIS). The CCD has a complicated driving mechanism, consumes a considerably power, and requires a multi-step photo process. Hence, the CCD has a disadvantage of a complicated fabricating process. Moreover, the CCD has difficulty in integrating a control circuit, a signal processing circuit, an analog/digital (A/D) converter and the like on and/or over a CCD chip, thereby being disadvantageous in downsizing a product.
As a next generation image sensor for overcoming the disadvantages of the CCD, attention has focused on the CIS. The CIS image sensor includes MOS transistors equal in number to the number of unit pixels. The CIS may be formed on and/or over a semiconductor substrate by CMOS technology that uses a control circuit, a signal processing circuit and the like as peripheral circuits. Thus, the CIS is the device adopting the switching system for sequentially detecting outputs of unit pixels by the MOS transistors, respectively. In particular, the CIS implements an image in a manner of forming a photodiode and a MOS transistor within a unit pixel and then sequentially detecting an electric signal of the corresponding unit pixel by switching. Since the CIS uses CMOS fabrication technology, it thereby has advantages of low power consumption, a simple fabricating method due to the reduced number of photo process steps and the like. Moreover, the CIS is able to integrate a controller, a signal processor, an A/D converter and the like on a CIS chip, thereby having advantage in downsizing a product. Therefore, the CIS is widely used for the various applied field of a digital still camera, a digital video camera and the like.
Example FIG. 1 is an equivalent circuit diagram of a unit pixel of a CIS that includes a single photodiode (PD) and four MOS transistors. A unit pixel of a CIS includes photodiode (PD) generating photocharges by receiving light, transfer transistor Tx transferring the photocharges collected in photodiode (PD) to floating diffusion region (FD), rest transistor Rx setting a potential of floating diffusion region (FD) to a specific value and resetting floating diffusion region (FD) by discharging electric charges, drive transistor Dx playing a role as a source follow buffer amplitude, and select transistor Sx playing a switching role to enable addressing. A load transistor is provided outside the unit pixel to enable an output signal to be read.
Example FIG. 2 is a cross-sectional diagram of the CIS illustrated in example FIG. 1. As illustrated in example FIG. 2, the CIS includes a field oxide layer formed on and/or over semiconductor substrate 11 defined into a sensing part and a driving part to define an active area, plurality of photodiodes 12 formed in the active area of semiconductor substrate 11, and plurality of transistors 13 formed on and/or over the active area of semiconductor substrate 11. First insulating interlayer 14 is formed on and/or over substrate 11 including the sensing part having photodiode 12 and transistor 13 and a peripheral driving part. First metal line M1 is formed on and/or over first insulating interlayer 14. Second insulating interlayer 15, second metal line M2, third insulating interlayer 16, third metal line M3, fourth insulating interlayer 17, fourth metal line M4 and protective layer 18 are sequentially formed on and/or over first metal line M1.
Contact holes and contact plugs are formed in insulating interlayers 14, 15, 16 and 17 between metal lines M1, M2, M3 and M4 to electrically connect metal lines M1, M2, M3 and M4, respectively. Second metal line M2, third metal line M3 and fourth metal line M4 are provided in the peripheral driving part, thereby not affecting light incident on and/or over photodiode 12. R/G/B color filter layer 19 is formed on and/or over protective layer 18 of the sensing part to implement a color image. An array of microlenses 20 is formed on and/or over and spatially corresponding to color filter layer 19. Microlens 20 obtains a specific curvature in a manner of coating photoresist, patterning the photoresist to remain on and/or over photodiode 12 only and then reflowing the photoresist by baking. Microlens 20 plays an important role in condensing an incident light on and/or over photodiode 12.
However, in the process for fabricating such a CIS, after protective layer 18 has been formed, before color filter layer 19 and microlens 20 are formed, thermal treatment is performed at a temperature of about 450° C. to enhance the dark characteristic. In particular, when a dark image is captured, a white dot-type can be generated. The cause of the white dot-type is explained as follows. First, the photodiode is formed by implanting impurity ions and an ion beam is used for an etch process for forming a thin film transistor or a metal line. The impurity ion implantation or the ion beam may charge a silicon substrate surface with electrons. If the surface is charged with the electrons, the white dot-type is generated on a black screen. This is called a dark defect. To settle the dark defect, after protective layer 18 has been formed, before color filter layer 19 and microlens 20 are formed, the thermal treatment is performed at the temperature of about 450° C. Through the thermal treatment, hydrogen atom of silane gas (SiH4) used in depositing the insulating interlayer or the like pushes out to replace the charged electrons at the surface of the silicon substrate, whereby the dark characteristic is enhanced.
However, such a CIS fabricating method has the following problem. First, if the thermal treatment is performed to enhance the dark characteristic, thermal stress is generated from the metal line in the structure where the metal line, the insulating interlayer and the protective layer diffuse into a single layer to destroy the protective layer on and/or over the uppermost metal line. Therefore, the protective layer function for protecting devices is degraded or otherwise lost.